"I can easily see still in my mind’s-eye the beautiful clusters of these berries as they appeared to me..., when I came upon an undiscovered bed of them... – the rich clusters drooping in the shade there and bluing all the ground" -- Thoreau

Those articles reference dust as a possibility, but I'd say that is probably the least likely of the choices. Particularly given the very high altitude, consistent location over the same place, and whitish color.

Dust is very unlikely given that the feature does not persist all day, and the increasing visibility into the blue. The single scattering albedo of water ice is essentially 1 through the optical regime, while airborne dust follows the properties of the surface (more or less in terms of albedo). See the attached MRO/MARCI "daily global" normalized-radiance maps in Band 7 (330 nm): March 20 and 21 (UT). These maps are "quick and dirty" and more importantly, represent the the local time of ~14h-16h30 across each strip (at least away from the polar regions).

If the feature was caused by dust, there would have to be a lot of it (from a light-scattering perspective), and it would be expected to be present in the afternoon (i.e., attached maps; though the location of the feature is inconveniently split across the boundaries of the cylindrical projection used).

What makes the feature interesting to me is its southern extent, particularly with respect to the so-called aphelion cloud belt.

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